1. Field of the Invention
The invention relates to a device for treating gases by means of a surface plasma, in the presence of a catalyst.
The fields of use of the present invention comprise, in particular, the degradation of pollutants likely to be contained in gases, the reforming, and the upgrading of gases.
2. Description of Related Art
In the field of gas treatment, methods using a plasma may result in being particularly advantageous since they enable elimination of pollutants at ambient temperature for a low energy cost when the elements are present in the gases in minute quantities. They may also lower the temperature of reaction between two gaseous compounds and/or lower the power necessary to carry out a reaction between two compounds. The plasmas may be volume or surface plasmas at atmospheric pressure.
In the case of atmospheric plasmas, the dielectric barrier discharge or DBD technology is generally implemented. This technology comprises applying an A.C. signal between two electrodes, a dielectric substrate being interposed between the two electrodes to avoid the forming of an electric arc (FIG. 1).
In the case of volume DBD plasmas, the space between electrodes is limited to a few millimeters due to the fact that voltage necessary to generate the plasma increases with the inter-electrode space (FIG. 1) and that the thickness of the dielectric substrate is linked to its dielectric strength. The inter-electrode spacing depends, in particular, on the nature of the dielectric substrate and on the applied voltage. The dielectric thickness conventionally is between 3 and 5 mm and the free space for the flowing of gases is of the same order of magnitude, which generates a significant head loss. In such a configuration, and as described in document US 2002/0070127, a catalyst may be introduced in the plasma area by deposition on the surface opposite to the electrode. The electrode itself may also be used as a catalyst in the case where it is made of an electrically-conductive material.
The DBD technology may also be implemented to generate a surface plasma. The plasma is then created in the vicinity of the surface of a dielectric substrate. The two electrodes are arranged on this dielectric substrate, on either side of the main surfaces of the dielectric substrate (FIG. 2).
The plasma area can thus be adjusted according to the inter-electrode space. In this configuration, the distance between dielectric substrates is independent from the discharge parameters. Such surface plasmas create an acceleration of the gas speed near the electrodes, as described in document U.S. Pat. No. 7,380,756. The multiplication of electrodes on the surface enables the creation of jet effects perpendicularly to the surface, as described by Bénard et al. (“Thin Solid Films”, Vol. 516, pp. 6660-6667, 2008).
Document FR 2918293 provides using such surface plasmas for the degradation of pollutants in a gaseous atmosphere. It describes the use of a photocatalyst (TiO2) arranged in the form of a thin layer in contact with the dielectric substrate in the inter-electrode space, such a catalyst being intended to select the decomposition products. In this case, the catalyst thus cannot be an electric conductor such as a metal, to avoid a strong decrease of the plasma area.
The present invention relates, in particular, to a device enabling association of the generation of a surface plasma with a wide range of catalysts, for the treatment of gases, in particular the degradation of pollutants, the reforming, and the upgrading of gases.
The present invention enables improved conversion of gases, but also decreased head losses, while providing the lowest possible power consumption and the lowest possible temperature.